The present invention relates to infrared imagers.
It is highly desirable to have an infrared area array imager which will provide a detailed image at wavelengths down to the limits of the available long-wavelength atmospheric window, i.e. at wavelengths of 8-12 microns.
The detection of such long wavelengths, if it is to be done at moderate cryogenic temperatures (e.g. at liquid nitrogen rather than liquid helium temperatures), is preferably done using a very narrow bandgap semiconductor, such as Hg.sub.1-x Cd.sub.x Te. (Such alloys, where X ranges from 0 to 1, are generally referred to as "HgCdTe".) This pseudo-binary alloy, if it has a composition such as x=0.2, will have a bandgap small enough (0.1 eV) to be bridged by 12 micron photons.
Conventional approaches using HgCdTe area arrays have typically used HgCdTe photodiodes as optical detectors, and have interconnected these photodiodes with silicon processing stages.
However, if any sizable array of infrared detectors is used, it can be a very difficult problem to get the raw output of the infrared detectors over to the silicon processor stages. That is, there are many applications for which a two-dimensional focal plane array larger than 100.times.100 would be desirable. In such applications, it is very difficult to connect so many infrared detector locations to silicon processing stages without greatly degrading the duty cycle of the detector stages. In particular, where photo-diode detector cells are used, the silicon processing circuitry required is rather complex, and a merely reasonable pitch in the infrared imaging plane (e.g. 0.002" pixel spacing center-to-center) would impose stringent requirements on silicon geometry, if the same pitch must be followed in the silicon processing stages.
One method which has been used in the prior art to connect infrared detector sites to silicon processors is a hybrid approach, in which the infrared detector cells are bump bonded, in many separate operations, onto a silicon carrier. This is an expensive low-yield operation.
Thus it is an object of the present invention to provide an infrared focal plane array imager architecture which permits direct connection from an infrared detection site to silicon processing stages. It is a further object of the present invention to provide an infrared focal plane array imager architecture which permits very high duty cycles at the infrared sensor sites.
An important difficulty in the development of usable long-wavelength infrared imaging arrays has been the stringent requirements placed on materials quality. That is, to achieve acceptable device characteristics (e.g. large well capacity, high sensitivity, low dark current, large dynamic range, etc.) using the prior art techniques, material having inherent carrier concentrations from the low 1014s per cc on down is conventionally required. Such material is very difficult to fabricate reproducably.
Thus it is an object of the present invention to provide a long-wavelength infrared imager which does not impose stringent material requirements. It is a further object of the invention to provide a long-wavelength infrared immager which does not require inherent carrier concentrations below 5.times.10.sup.14 per cubic centimeter.
A further difficulty in long-wavelength imaging is that the background flux is very high. That is, the peak black body wavelength at room temperature is very close to 12 microns, so that stray (near-field) long-wavelength radiation is likely to be generated by the infrared imaging optics of the imaging system. Moreover, very little of a field of view will usually be truly dark. That is, in thermal imaging the brightness variation within an image will be due to typically small variations in temperature and to variation in the black-body coefficients. These effects produce a dynamic range within a scene which is relatively small. Little of the photon flux carries information since most of the flux merely corresponds to the high average temperature of the scene. This is frequently handled, in conventional long-wavelength imaging systems by a "skimming" operation in which the signal from each pixel is thresholded to eliminate the effect of the background flux. However, this thresholding operation does not improve the signal-to-noise ratio, and may worsen it, since the noise component due to fluctuation in the background flux remains.
Thus it is an object of the present invention to provide a long-wavelength detector which provides a very good signal-to-noise ratio, even in the face of substantial long-wavelength background flux.
The present invention teaches an MIS detector. This is a broadly new concept in infrared imaging array architecture. An insulated gate is provided over an expanse of a narrow-bandgap semiconductor, such as HgCdTe. The gate is charged up, to create a depletion well in the semiconductor beneath the gate, and then floated. This depletion well then collects carriers from photon-generated pairs. At the end of the collection cycle, the voltage on the MIS gate is sensed, which effectively measures the number of carriers accumulated in the depletion well. The gate voltage is then controlled to collapse the well, recombining the stored carriers, and a new depletion well is then created to begin a new detection cycle. The charge collected from the MIS gate is not immediately provided to the final output signal, but instead is averaged by another capacitor located in the silicon immediately below the HgCdTe pixel site. By recursively averaging several outputs from the MIS capacitor in the silicon capacitor, a greatly improved signal-to-noise ratio at the output of the silicon capacitor is obtained. Conventional methods are then used to readout the charge from the silicon averaging capacitor.
According to the present invention there is provided:
An infrared imager comprising:
a plurality of detector locations, each said detector location comprising PA1 said semiconductor being positioned to receive infrared photons; PA1 means, connected to said respective storage gates, for biasing said semiconductor at said respective detector locations into depletion, whereby optically generated carriers can collect in the depletion region defined beneath said respective storage gate; and PA1 means, connected to said storage gate, for detecting the potential of each said respective storage gate after said semiconductor beneath said storage gate has been exposed to infrared photons for a predetermined length of time.
a substrate comprising a narrow-band-gap semiconductor, PA2 an insulator on said semiconductor, PA2 and a conductive storage gate on said insulator,